Ecommerce Scenario Modelling: A Sydney Virtual CFO Method
A virtual CFO method for ecommerce scenario modelling. Three cases, one decision, built on the three levers that actually move a Sydney DTC business.
Most scenario models exist to admire uncertainty. A useful one exists to force a decision. For a $10M ecommerce business, the decision is almost always the same: how much stock to commit for the next peak, and how much cash that commitment puts at risk. This is a virtual CFO’s method for building three scenarios that answer that question rather than decorating it.
Published: July 2026
A three-case model, base, downside, upside, is only worth building if each case changes what you would do. If the three columns produce three interesting numbers and no different action, the model failed. So the first step is not to open a spreadsheet. It is to name the decision.
For a $10M direct-to-consumer brand, the recurring decision is the peak-season inventory buy. You commit cash to stock months before customers pay you, the buy is largely irreversible once the purchase order is placed, and the downside of getting it wrong is either stockouts through your best trading weeks or a warehouse full of capital you cannot sell. That is a decision worth modelling three ways. Once you have named it, the 90-Day Number can build the model around it, or you can build it yourself with what follows.
An ecommerce business has dozens of variables and three that actually move the outcome. Flex these three and hold the rest, or the model becomes noise.
The first is demand: units sold through the period. This is the lever with the widest range and the biggest consequence, because it drives both revenue and how much of the inventory buy you actually clear.
The second is contribution margin: what each order contributes after landed cost, fulfilment, payment processing, returns, and variable acquisition. If you have not built this yet, start with ecommerce unit economics, because scenario modelling on a margin you have not verified is modelling on a guess.
The third is inventory: how much stock you commit and when the cash goes out. This is the lever you most directly control and the one that most directly threatens cash.
Everything else, rent, salaries, software, holds flat across the three cases. Fixed costs are fixed; that is the point of calling them fixed. Flexing them to make a scenario look better is how founders talk themselves into a bad buy.
Take a brand doing $10M in annual revenue, with a verified contribution margin of 22 per cent and a fourth-quarter peak that historically carries 35 per cent of annual sales. The buy decision is how much stock to commit for that quarter. Build three cases on demand, holding the cost structure constant.
Laid side by side, the three cases turn an instinct into an arithmetic. The founder’s real question is how much inventory to commit, and the model reframes it as a trade-off between two costs: the cost of overbuying (cash trapped in unsold stock in the downside) and the cost of underbuying (contribution left on the table in the upside).
If the downside case threatens the cash position enough to endanger the business, the answer is to commit less stock and accept the risk of a capped upside. If the balance sheet can absorb the downside comfortably, the answer is to buy closer to the upside and capture the demand. The model does not make the decision. It makes the decision legible, so the founder is choosing between quantified outcomes rather than guessing.
Revenue and contribution are the headline numbers in each case, but the line that decides whether you survive a bad peak is cash. An ecommerce business can be profitable on paper through a downside quarter and still run out of money, because the stock was paid for in September and the disappointing sales dribbled in through December and January.
So every scenario should carry a cash view, not just a P&L view. The question is not only “what is contribution in the downside” but “what is the lowest the bank balance goes, and on what date”. A 13-week cashflow forecast is the natural companion to the scenario model here: the scenario model sizes the buy, the 13-week tracks the cash consequence week by week as it plays out.
A scenario model built once before the buy and never touched again is a historical document by November. The value is in updating it as real demand data arrives. Two or three weeks into the peak, you know which case you are tracking toward, and the model tells you whether to reorder, hold, or start discounting to clear stock before it ages into dead capital.
This is the difference between a scenario model and a forecast. The forecast is a single expected path. The scenario model is a set of prepared responses, so that when demand surprises you, you already know what you will do. For a broader treatment of rolling this discipline across the whole business, see rolling forecasts versus the annual budget.
How many scenarios should I build?
Three: base, downside, upside. Two does not show you the spread, and more than three usually means you are flexing variables that do not change the decision. If a fourth case would lead to a meaningfully different action, build it. Otherwise it is decoration.
Which variables should I flex?
For ecommerce, demand, contribution margin, and inventory commitment. Hold fixed costs constant across cases. The discipline of flexing only the three levers that move the outcome is what keeps the model useful rather than overwhelming.
How is scenario modelling different from a budget?
A budget is a single expected path, usually set once a year. A scenario model is a set of prepared responses to different outcomes, built to inform a specific decision. The budget tells you what you expect; the scenario model tells you what you will do if you are wrong.
How far out should the model run?
Match it to the decision. For a peak-season inventory buy, model through the peak and the recovery into the new year, so the cash trough is fully visible. For a longer-horizon decision, extend it, but resist the urge to model three years of a business that cannot see three months of cash clearly.
What is the most common mistake in ecommerce scenario modelling?
Flexing the cost base to rescue a scenario. Founders faced with a scary downside case often quietly assume they would cut marketing or overheads, which makes the downside look survivable. Model the honest downside first, with costs held, then decide separately what you would cut. Do not build the cut into the base assumptions.
Do I need a virtual CFO to build this?
No, the framework above is usable on your own. A virtual CFO adds value by building it once with verified margins and a proper cash view, and by tying it to a 13-week forecast so the scenario translates into weekly cash tracking. That is one of the deliverables the 90-Day Number can produce.
What if my margins are not verified?
Then verify them before you model. Scenario modelling amplifies whatever margin assumption you feed it, so an unverified contribution margin produces three confidently wrong cases. Build the unit economics first, then model on top of them.
Sydney Virtual CFO is a Sydney-based virtual CFO service for founders running $2M to $15M businesses across SaaS, ecommerce, professional services, construction, and other low-volume, high-value industries. We deliver fixed-scope CFO engagements with a named deliverable on day 90: a 13-week cashflow forecast, a fundraise-ready financial model, a unit economics build, or a board reporting pack you can run on your own.
Our front-door product, the 90-Day Number, is fixed scope at $17,850 plus GST. We are one of the few project-based virtual CFOs in Australia, in a market built almost entirely on monthly retainers. No retainers without a deliverable. No 80-page reports. No theatre.
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This content is general information only, written for Australian founders running businesses in the $2M to $15M revenue range. It does not constitute tax, financial product, investment, or legal advice and should not be relied on as such. The work referenced is led by a Chartered Accountant (CA ANZ), but Sydney Virtual CFO is not a licensed tax agent, not a licensed financial adviser, and not authorised to provide personal financial advice. Tax obligations, accounting treatments, fundraise terms, and statutory requirements depend on your individual circumstances. For advice specific to your business, contact the team directly or consult a registered tax agent, licensed financial adviser, or qualified lawyer. Information was current at the time of publication and may change without notice. We review and update guides periodically.
